Plaster board has been widely used as a building material for the interior from the viewpoints of fire resistance, sound insulation, workability, and economy. The “plaster board” is a board-shaped body in which a plaster-based core (plaster core) is covered with a base paper for plaster board (simply, referred to as a base paper below). Generally, a plaster board is fabricated by the following steps (1)-(5).
(1) A calcined plaster, an adhesion assistant, a set accelerator, foam, and other additives, and water, etc., are kneaded by a mixer so as to obtain a calcined plaster slurry (simply, referred, to as a slurry below).
(2) While the plaster slurry is provided onto a lower-side (front surface-side) base paper (a front surface covering base paper) that is supported and moves on a long moving belt, opposite parts of the front surface covering base paper are folded so that margins of those parts reach to a back-surface side of the plaster board.
(3) At the back-surface side of the plaster board, margins of another upper-side base paper (a back surface covering base paper) are overlapped with the margins of the folded parts of the front surface covering base paper, wherein paste is applied on the margins of the back surface covering base paper.
(4) The slurry spread by and covered with the front surface covering base paper and the back surface covering base paper through a molding machine is molded into a board shape through the molding machine.
(5) After the slurry is hardened, the hardened plasterboard is roughly cut, subjected to forced-drying, and cut into a product size.
As a representative base paper for plaster board, a front surface covering base paper located on one surface of a plaster core and a back surface covering base paper located on the opposite surface of the plaster core are provided. The front surface covering paper covers opposite side-surfaces of the plaster core and the margins of the front surface covering base paper are pasted with the margins of the back surface covering base paper.
In order to save on the weight of the core, conventionally a cellular porous plaster core has been fabricated by introducing foam into a plaster core. For example, the cellular porous plaster core has been fabricated by adding a foaming agent into plaster slurry.
However, a core formed from only foamed plaster has two disadvantages. First, the hardened cellular porous plaster core is comparably fragile, and the cellular porous core is easy to crack and break as a nail is driven into the cellular porous plaster core under construction. The term of “nail” used herein means a component for holding a plaster board on a member, including not only a nail used for fixing the plaster board to a wooden substrate under construction of a building but also a machine screw (or a screw thread) used for fixing the plaster board to a metallic substrate (stud). Second, the cellular porous plaster core does not necessarily adhere to the base paper.
Regarding such problems, Japanese Laid-Open Patent Application No. 4-505601 discloses a plaster board characterized in that a single foamed slurry containing a comparably “easier broken foam” than foam in the conventional foamed slurry is deposited on a base paper and dried, an interface area containing fibers originating from the base paper and plaster originating a plaster core between the base paper and a surface of the plaster core and a layer containing no fiber adjacent and approximately parallel to the interface area are included, which layer includes plaster having fewer voids and greater density than in the central parts of the plaster core, which central part of the plaster core includes a multi-layer core having a plurality of discontinuous voids equal to or less than 1705 per square centimeter and approximately uniformly distributed throughout.
According to the conventional technique, a “separation” defect such that the paper on one surface of the plaster board is easily peeled from the plaster board can be significantly improved by a thin layer proximate to this paper and containing no fiber of the paper, and the strength of the plaster board can be significantly reduced by forming voids fewer and larger than the conventional voids in the hardened plaster board.
Additionally, in order to improve the adhesive property between the plaster core and the base paper, for example, Great Britain Patent No. 741140 discloses the technique of molding a plaster board by applying high-density slurry containing no foam on the adhesion surface of the base paper uniformly and thinly by a roller and providing low-density slurry containing foam onto the high-density slurry and then covering the low-density slurry with the base paper or providing another base paper on the surface of which the high density slurry containing no foam is applied uniformly and thinly by a roller.
Furthermore, Japanese Laid-Open Patent Application No. 5-148001 discloses a technique of molding a plaster board by providing high-density slurry containing an adhesive between a spreader roll and an idler roller, transferring slurry that adheres to the surface of the spread roller to the adhesive surface of the base paper so as to provide a thin plaster stratum on the adhesive surface, and providing low-density slurry containing foam onto the plaster stratum.
Next, drying of the plaster board is given as a large factor on fabrication in regard to adhesion of the plaster board. That is, in regard to the plaster board in a forced-drying process, generally, a drying rate in regard to a side edge part or side edge part area of the plaster board is comparably faster than a drying rate in regard to a central part of the plaster board. For this reason, in the side edge part or side edge part area, the degradation in the strength and the failure of adhesion (that is, dry-out) due to excessive drying easily occurs.
As a method of preventing the dry-out, for example, U.S. Pat. No. 2,762,738 discloses a technique of preventing the dry-out by providing high-density slurry from sub-mixers provided above both margins of the front surface covering base paper respectively onto the side edge portions of the slurry provided from a main mixer on the front surface covering base paper so as to form a plaster board, and making the core density at both side edge parts of the plaster board be larger than the density in the central part.
Similarly, Japanese Laid-Open Patent Application No. 9-511702 discloses an apparatus for forming a coating and side edge portions of a comparably high-density plaster slurry on a covering base paper by means of a coating roller rotating opposite to the moving direction of the covering base paper, and then forming a core including comparably low-density plaster between the side edge portions on the coating.
The length of the coating roller in the axial directions is smaller than the width of the covering base paper. Accordingly, the covering base paper with a normal width extends over the ends of the coating roller, and some slurry leaks out near both ends of the coating roller on the margins or the outside of the covering base paper. The base paper with a coating film is transferred on a deck along a transportation route of the base paper and an additional high-density slurry is poured onto the margins of the covering paper so as to form a rigid edge (hard edge) part. The apparatus for forming a core is provided with means for providing comparably low-density plaster slurry between the side edge portions on the coating.
However, in regard to a plaster board including a multi-layer core formed from a single foamed slurry disclosed in Japanese Laid-open Patent Application No. 4-505601, plaster is hardly ever or never stuck to the base paper due to density deficiency and the dry-out phenomenon in the side edge parts of the plaster board in formation of the plaster board and subsequent forced-drying, so that the defect of no base paper remaining on the plaster panel may occur. In order to reduce such a dry-out phenomenon, for example, as disclosed in Japanese Laid-Open Patent Application 11-501002, there is a method of preparing another high-density slurry and providing it onto the side edge parts of the plaster board so as to mold the plaster board. In this case, the area of the hard edge part originating from the high-density slurry becomes too large. Accordingly, when a nail is driven into the plaster board under construction, workability and/or subsequent finishing may be provided with such a disadvantage that the nail cannot sufficiently enter into the plaster board or the nail can enter but the head of the nail projects, at a nailing position along the side edge parts of the plaster board in the longitudinal directions.
In the conventional techniques disclosed in Great Britain Patent No. 741140 and Japanese Laid-Open Patent Application No. 5-148001, a plaster containing no foam may be too hard to practically function as an optimal material for a plaster board. Also, in a roll coater used in the fabrication method of these plaster boards, since the rotational direction of a spreader roll is the same as the transportation direction of a base paper, a slurry spread by the roller tends to adhere to the roller surface due to the tackiness of the slurry. As the result, the thickness of a thinly applied slurry layer is not constant along the transportation direction of the base paper. Furthermore, since the slurry adhering to the surface of the spreader roll is a reaction hardening type, in which calcined plaster as the main component contacts with water to harden, the slurry is hardening in progression and the gap between a spreader roll and an idler roller becomes narrower, during the rotation of the spreader roll. Consequently, the amount of the slurry provided onto the surface of the spreader roll and transported is reduced, so that the thickness of a thinly applied slurry layer varies with time so as to be not constant. On the other hand, after application of the slurry, even if a lump removing plate is provided across the entire roller along the longitudinal directions in order to remove a residual slurry on the roller surface, the removed slurry falls on the surface of a thinly applied slurry layer from the lump removing plate and consequently it is difficult to provide a slurry layer with a uniform thickness. Moreover, as slurry adhering to the side surface of the roller, etc., hardens and grows so as to make contact with a base paper, slurry causes cutting of the paper etc. As a result, such a problem that the production line has to be stopped persists.
Also, in the conventional technique disclosed in U.S. Pat. No. 2,762,738, since low-density and high-density slurries are separately prepared by a main mixer and a sub-mixer so as to be provided onto the central part and the margin of a base paper, respectively, fabrication processes and process management for the plaster board are cumbersome and complex, and the size of a hard edge part of the fabricated plaster board is not sufficiently controlled.
Similarly, in the conventional techniques disclosed in Great Britain Patent No. 74110 and Japanese Laid-Open Patent Application No. 5-148001, since two slurry mixers for preparing coating slurries for two base papers are used in addition to a main mixer, a configuration is required in which the controls of the mixers are difficult. Additionally, combining a plaster core and a base paper is disclosed by sticking a dense gypsum plaster slurry, particularly slurry containing a binder such as starch, etc. in the latter patent application, to the base paper by an applicator roller. However, there is no description for problems in regard to the dry-out and hard edge.
Additionally, the slurry in the main mixer used in the apparatus in Japanese Laid-Open Patent Application No. 9-511702 contains plaster, water, a forming agent, and a stabilizer, etc. and has a composition for forming the comparably low-density plaster slurry. Also, almost all of the air bubble contained in the slurry poured into the roller coating are removed by providing the foamed slurry from the main mixer through a pipe line to a high-speed beater and stirring the slurry by using a rapidly rotating wing of the high-speed beater. Thus, the slurry provided onto the roll coater or the margin of the covering base paper is a comparably high-density slurry. Accordingly, first in order to form a single foamed slurry, since the calcined plaster slurry and the foam are considerably stirred in the mixer, this conventional technique causes the problems of loss of the foam and ununiformity in the size of the air bubbles and the voids. Also, since defoaming capability of the high-speed beater has a limitation, it is difficult to provide the desired high-density slurry continuously and stably. Moreover, foam removed from the slurry provided onto the roll coater is wasted and the foaming agent and labor spent for forming foam are also wasted. Additionally, since the additional high-density slurry (hard edge slurry) is provided onto the margins of a cover sheet (base paper) provided with a slurry coating by the spreader roll through the pipe line, such problems that it is difficult to control the size of the hard edge part and it is difficult to drive a nail into the plaster board easily occur.